"These findings provide a molecular, pathological signature of these disorders, which is a large step forward," said senior study author Daniel Geschwind.

"The major challenge now is to understand how these changes arose," added Geschwind, director of the University of California, Los Angeles' Center for Autism Research and Treatment.

"We show that these molecular changes in the brain are connected to underlying genetic causes, but we don't yet understand the mechanisms by which these genetic factors would lead to these changes," Geschwind said in a university news release.

While there was significant overlap in gene expression patterns associated with conditions such as autism and schizophrenia, people with major depression had gene expression patterns not seen in other types of mental illness, the researchers said.

Now that the researchers have some understanding of causes, the next step is to understand the underlying mechanisms "so as to develop the ability to change these outcomes," Geschwind said.